Bacterial Reaction Centers with Modified Tetrapyrrole Chromophores

Abstract

II. Norris, James R., Date. QP517.P45P46 1993 '3342-dc20 92-43633This book is printed on acid-free paper.© Academic Press, Inc. References 78 Genetic Analysis of Photosynthetic Membrane Biogenesis in Rhodobacter sphaeroides I. IntroductionSeveral different methods are available for preparing reaction centers with nonnative chromophores: (1) (re)assembly from the Polypeptides, pigments, and other components such as quinones, iron, and possibly others; (2) exchange of chromophores into native or partly denatured reaction centers; (3) modification of chromophores in native or partly denatured reaction centers by chemical or enzymatic means; (4) modification of binding sites to incorporate different chromophores, for example, by mutagenesis, and (5) mutagenic modifications of the pigments biosynthetic pathways. With the exception of the last one, all these approaches have been used on different Complete reconstitution of the complexes (Method 1) from the isolated constituent Polypeptides and suitably modified pigments is likely to have the greatest potential, but to date has been impossible with reaction centers. This technique has been applied successfully to the core light-harvesting complexes (LHCI) of purple bacteria (Miller et al, 1987;Ghosh et al, 1988;Parkes-Loach et al, 1988 Chang et al, 1990a,b;Heller and Loach, 1990) and to the peripheral light-harvesting chromoprotein LHCII-ß of higher plants (Plumley and Schmidt, 1987). The method has been extended by Paulsen etat. (1990Paulsen etat. ( ,1991 to the full-length and modified LHCII-ß precursor Polypeptides expressed in and isolated from Escherichia coli and by Schmidt et al. (1991) to other green plant peripheral light-harvesting complexes.Site-directed mutagenesis of the binding pockets (Method 4) has been used extensively to exchange bacteriochlorophylls (Behls) with their metalfree derivatives, for example, bacteriopheophytins (Bphes) and vice versa in reaction centers (RCs) from Rhodobacter (Rb.) capsulatus and Rb. sphaeroides (Table I)-With only a Single exception, whenever the histidine(s) ligating the central magnesium atom(s) of the Bchl(s) were replaced by hydrophobic amino acids, stably assembled RCs were obtained in which the Behls at the respective site(s) were replaced by Bphe to yield stably assembled and isolatable RCs (Bylina and Youvan, 1988; Coleman and Youvan, 1990;Robles et al, 1990;Schenck et al, 1990;Woodbury et al, 1990). In reversing this reaction, replacement of leucine or isoleucine next to the central N-4 cavity of Bphe with histidine resulted in binding of Behl at the respective site. Some results are available with amino acid residues of intermediate ligation strength. Serine or threonine selected binding of Behl at BA and BB (Bylina et al, 1990); glutamine (mutants H-L173Q and H-M200Q 2 ) at the sites of the special pair resulted in binding of Behl also (Bylina and Youvan, 1988). Omission of Bphe HL was found in a mutant of Rb. capsulatus in which part of the D helix of subunit L was replaced by the symmetrically equiv...

Aerobic anoxygenic photosynthetic bacteria with zinc-bacteriochlorophyll.

“…Because of the high structural similarity of Zn-BChl a to Mg-BChl a, there are similarities in absorption maxima in organic solvents between the two (Table 2), although Zn-BChl a is characterized by its absorption maxima blue-shifted as noted above. Artificially Zn-transmetalated BChl a has the identical spectral pattern (Scheer and Hartwich, 1995).…”

Energetics of the primary electron transfer reaction revealed by ultrafast spectroscopy on modified bacterial reaction centers

“…sphaeroides R26.1 are compared with modified RCs (R26*Phe-a) in which the bacteriopheophytin a (BPhe-a) molecules were exchanged to more than 95% by pheophytin a (Phe-a) by a procedure given in refs. [ 24,25 ] . The in vitro difference of the redox potential of BPhe-a and Phe-a of AU= 0.13 V [ 261 suggests a comparable rise of G( P+H, ) in the modified RC.…”

Radicals and Radical Pairs in Photosynthesis

“…Besides site-directed mutagenesis, chemical exchange of cofactors has become feasible and has been used in the past several years to investigate both the relevant binding pockets within the protein and the pigment-pigment interactions, which are mainly responsible for primary charge separation (104). While direct exchange of parts or all of the primary donor have not yet been achieved, it was proven that selective exchange of the two accessory BChl monomers on either side (A or B) is possible (105).…”